j:ffi-:cts of uKorc uts on nisTRiia'TioN of plants. 385 



that period, hence the causes must have been others than merely 

 chmatic. 



11. TllF DlSAPI'KARANCE OF A Sl'FCtlS I'ROM A LoC ALFl ^ OWING 

 TO CHANGED ENVIRONMENTAL CONDITIONS. 



The well-known (jrchid, called the '" Pride of Table Moun- 

 tain," z'i::., Disa unifloni, inhal)its the banks of permanent stream- 

 lets and waterfalls of the south-western movmtains, as well as 

 ledges and crevices of rocks, which receive sufficient moisture, 

 even in the dry season. Along one of the streams on Table 

 Mountain, t=/,cr., in the Arch Valley, 1 knew this disa to grow in 

 profusion, having counted, on one occasion, about -'6 years ago, 

 over 200 blooms within a distance of less than half a mile. When 

 visiting this locality last February 1 found, to my surprise, that 

 most patches of plants along the stream had disappeared, and that 

 this season had jiroduccd just fotu-teen flowers. One might feel 

 inclined to ascribe the paucity of flowers to the extremely dry 

 summer of 191 5, and there is no doubt that in many places, espe- 

 cially on cliffs, the plants did not flower as freely as in normal 

 years, owing to the absence of stifficient moisture. This stream, 

 however, carried \Aater, although less than in former years, and 

 the disappearance of the plants themselves from the greater 

 part of the banks of the stream cannot be ascribed to the paucity 

 of water in the stream, which began to assert itself only in the 

 beginning of the year. The cause is a difterent one. In the 

 course of years the stream had gradually eaten its bed deeper 

 into the soil, and while (at the time when I first visited it (about 

 the year 18K8), the banks were mostly less than a foot high, 

 I found the channel now in many places 2^ or 3 feet deep, 

 and the rushes (Resfiacerc) from both sides entangled to such 

 an extent that one had to move them aside in order to find 

 the bed of the stream. It is obvious that — (i) the upper part 

 of the bank would be too dry for supporting the life of a Disa 

 tinifora, which requires permanently moist soil all the year 

 through : and ( 2) that many plants which might have survived 

 in spite of the reduced supply of moisture were choked out by 

 being deprived of light. 



Whether, on an average, the stream did actually carry less 

 water in recent years than 25 years ago, cannot be definitely 

 stated, although I feel inclined to think that this is so, but the 

 main cause of the dying out of the plants, or, rather, the great 

 reduction in its numbers along this stream, is the deepening of 

 the channel. 



How far this deepening may have affected the composition 

 of the other vegetation of the valle}- is not possible to say, but 

 thai it had destroyed at least nine-tenths of the disa plants is 

 obvious from mv observations. 



